The invention relates to frequency converters. More particularly, the invention relates to a frequency converter arrangement with at least two successive heterodyne stages, a first heterodyne stage which converts an input signal with an adjustable first heterodyne frequency of a first superposition oscillator, and a second heterodyne stage which converts an input signal with a fixed second heterodyne frequency of a second superposition oscillator into a second intermediate frequency.
Frequency arrangements of this type are known; see, e.g., German patent documents Nos. DE 27 44 432 and DE 40 21 294 and Japanese Patent Abstracts 60-28330A and 61-252720A. In such arrangements, a frequency divider which can be set in integral division ratios is used as frequency divider. The reference oscillator is an oscillator that is tuned to a fixed frequency. The phase comparison takes place at a frequency that is obtained by dividing the frequency of the reference oscillator. This frequency determines the step width with which the receive frequency can be shifted. To achieve a step width of 25 kHz, for example, just such a reference frequency is required. Due to this low reference frequency, the multiplication factor is large and the maximum possible control bandwidth is small, which leads to relatively bad phase noise and high transient recovery times in the frequency change. Thus, these known arrangements are not suitable for frequency conversion arrangements in receivers or spectrum analyzers with a frequency resolution in the Hz range.
The state of the art in these devices involves the mixing of the first local oscillator with the harmonic of a good frequency reference and the synchronizing of the mixed signal with a synthesizer signal. Due to the independent synchronization of the first and second local oscillators, the phase noise of these two oscillators is added, whereby the first oscillator dominates, since it has the higher frequency and is tuned via a synthesizer. The difference of the phase noise between the highest receive frequency and a receive frequency near zero is approximately 6 dB, since the first local oscillator is typically tuned over an octave and the phase noise is determined by the multiplication factor to the reference. This is not sufficient for applications in the receive range of up to several GHz; a phase noise is expected which is proportional to the receive frequency.
The same is true for signal generators in which the output frequency is generated in two successive heterodyne stages with a fixed, or respectively, a variable heterodyne frequency (or: beat frequency), as is the case in modulable signal generators.